High fre­quen­cy (HF) radio fre­quen­cies are between 3 and 30 MHz. Also known as the decame­ter band or decame­ter wave as the wave­lengths range from one to ten decame­ters (ten to one hun­dred metres). Fre­quen­cies imme­di­ate­ly below HF are denot­ed medi­um fre­quen­cy (MF), and the next high­er fre­quen­cies are known as very high fre­quen­cy (VHF). The HF band is a major part of the short­wave band of fre­quen­cies, so com­mu­ni­ca­tion at these fre­quen­cies is often called short­wave radio. Because radio waves in this band can be reflect­ed back to Earth by the ionos­phere lay­er in the atmos­phere, called “skip” or sky­wave prop­a­ga­tion, these fre­quen­cies can be used for long dis­tance com­mu­ni­ca­tion, at inter­con­ti­nen­tal dis­tances. The band is used by inter­na­tion­al short­wave broad­cast­ing sta­tions (2.310 — 25.820 MHz), avi­a­tion com­mu­ni­ca­tion, gov­ern­ment time sta­tions, weath­er sta­tions, ama­teur radio and cit­i­zens band ser­vices, among oth­er uses.

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